Which is the relation between the specific impulse ${I_s}$ and the critical temperature ${T_c}$ attained in a rocket blast?

• A
  ${I_s} \ge {T_c}$
• B
  ${I_s} \propto T_c^2$
• C
  ${I_s} \ge T_c^{1/2}$
• D
  ${I_s} \ge 1/{T_c}$